Fix write_to_file function

Probably, because of a racing condition, sometimes the file was
written into inconsistent state which breaks the configuration and
the hardware-exporter service.

After adding a file lock and detaching the permission from writting,
the bug seems to be fixed

Closes: #305

src/service.py

@@ -1,5 +1,6 @@
 """Exporter service helper."""
 
+import fcntl
 import os
 from abc import ABC, abstractmethod
 from logging import getLogger
@@ -280,16 +281,14 @@ def write_to_file(path: Path, content: str, mode: Optional[int] = None) -> bool:
     """Write to file with provided content."""
     success = True
     try:
-        logger.info("Writing file to %s.", path)
-        fileobj = (
-            os.fdopen(os.open(path, os.O_CREAT | os.O_WRONLY, mode), "w", encoding="utf-8")
-            if mode
-            # create file with default permissions based on default OS umask
-            else open(path, "w", encoding="utf-8")  # pylint: disable=consider-using-with
-        )
-        with fileobj as file:
+        with open(path, "w", encoding="utf-8") as file:
+            # Apply a file lock
+            fcntl.flock(file.fileno(), fcntl.LOCK_EX)
             file.write(content)
-    except (NotADirectoryError, PermissionError) as err:
+
+        if mode:
+            os.chmod(path, mode)
+    except (NotADirectoryError, PermissionError, OSError) as err:
         logger.error(err)
         logger.info("Writing file to %s - Failed.", path)
         success = False

tests/unit/test_service.py

@@ -770,9 +770,10 @@ def setUp(self):
     def tearDown(self):
         pathlib.Path(self.temp_file.name).unlink()
 
+    @mock.patch("service.fcntl")
     @mock.patch("builtins.open", new_callable=mock.mock_open)
     @mock.patch("service.os")
-    def test_write_to_file_success(self, mock_os, mock_open):
+    def test_write_to_file_success(self, mock_os, mock_open, mock_fcntl):
         path = pathlib.Path(self.temp_file.name)
         content = "Hello, world!"
 
@@ -783,22 +784,25 @@ def test_write_to_file_success(self, mock_os, mock_open):
 
         mock_open.assert_called_with(path, "w", encoding="utf-8")
         mock_file.write.assert_called_with(content)
+        mock_os.chmod.assert_not_called()
+        mock_fcntl.flock.assert_called_once()
 
-    @mock.patch("service.os.open", new_callable=mock.mock_open)
-    @mock.patch("service.os.fdopen", new_callable=mock.mock_open)
+    @mock.patch("service.fcntl")
+    @mock.patch("builtins.open", new_callable=mock.mock_open)
     @mock.patch("service.os")
-    def test_write_to_file_with_mode_success(self, mock_os, mock_fdopen, mock_open):
+    def test_write_to_file_with_mode_success(self, mock_os, mock_open, mock_fcntl):
         path = pathlib.Path(self.temp_file.name)
         content = "Hello, world!"
 
-        mock_file = mock_fdopen.return_value.__enter__.return_value
+        mock_file = mock_open.return_value.__enter__.return_value
 
         result = service.write_to_file(path, content, mode=0o600)
         self.assertTrue(result)
 
-        mock_open.assert_called_with(path, mock_os.O_CREAT | mock_os.O_WRONLY, 0o600)
-        mock_fdopen.assert_called_with(mock_open.return_value, "w", encoding="utf-8")
+        mock_open.assert_called_with(path, "w", encoding="utf-8")
         mock_file.write.assert_called_with(content)
+        mock_os.chmod.assert_called_with(path, 0o600)
+        mock_fcntl.flock.assert_called_once()
 
     @mock.patch("builtins.open", new_callable=mock.mock_open)
     def test_write_to_file_permission_error(self, mock_open):